Liquid Crystal Technology

Summary

Liquid crystal devices are the energy efficient, low-cost displays used in a variety of applications in which information or images are presented. The operation of the devices is based on the unique electrical and optical properties of liquid crystal materials.

Definition and Basic Principles

Liquid crystal technology is the use of a unique property of matter to create visual displays that have become the standard for modern technology.

Originally discovered as a state existing between a solid and a liquid, liquid crystals were later found to have applications for visual display. While liquid crystals are less rigid than something in a solid state of matter, they also are ordered in a manner not found in liquids. As anisotropic molecules, liquid crystals can be polarized to a specific orientation to achieve the desired lighting effects in display technologies.

Liquid crystals themselves can exist in several states. These range from a well-ordered crystal state to a disordered liquid state. In-between states are known as the smectic phase, which have layering, and the nematic phase, in which the separate layers no longer exist but the molecules can still be ordered.

Liquid crystal displays (LCDs) at this point typically use crystals in the nematic state. They also use calamitic liquid crystals, whose rodlike shape and orientation along one axis allow the display to lighten and darken.

Over time, researchers have made advances in the materials used for LCDs and the route of power for manipulating the crystals, allowing for the low-cost, high-resolution, energy-efficient displays that have become the dominant technology for displays such as computers and television sets. Future research should allow for improvements in the response time of the displays and for better viewing from different angles.

Background and History

Liquid crystals were discovered by Austrian botanist and chemist Friedrich Reinitzer in 1888. While working with cholesterol, he discovered what appeared to be a phase of matter between the solid (crystal) state and the liquid state. While attempting to find the melting point, Reinitzer observed that within a certain temperature range he had a cloudy mixture, and only at a higher temperature did that mixture become a liquid. Reinitzer wrote of his discovery to his friend, German physicist Otto Lehmann, who not only confirmed Reinitzer's discovery—that the liquid crystal state was unique and not simply a mixture of solid and liquid states—but also noted some distinct visual properties, namely that light can travel in one of two different ways through the crystals, a property known as birefringence.

After the discovery of liquid crystals, the field saw a lengthy period of dormancy. Modern display applications have their roots in the early 1960s, in part because of the work of French physicist and Nobel laureate Pierre-Gilles de Gennes, who connected research in liquid crystals with that in superconductors. He found that applying voltages to liquid crystals allowed for control of their orientation, thus allowing for control of the passage of light through them.

In the early 1970s, researchers, including Swiss physicist and inventor Martin Schadt at the Swiss company Hoffman-LaRoche, discovered the twisted-nematic effect—a central idea in LCD technology. (The year of invention is typically said to be 1971, although patents were awarded later.) The idea was patented in the United States at the same time by the International Liquid Xtal Company (now LXD), which was founded by American physicist and inventor James Fergason in Kent, Ohio. (Fergason was part of the Liquid Crystal Institute at Kent State University. The institute was founded by American chemist Glenn H. Brown in 1965.) Licensing the patents to outside manufacturers allowed for the production of simple LCDs in products such as calculators and wristwatches.

In the 1980s, LCD technology expanded into computers. LCDs became critical components of laptop computers and smaller television sets. With research continuing on liquid crystals into the twenty-first century, LCD televisions overtook cathode ray tubes (CRTs) as the dominant technology for television sets.

How It Works

LCDs have a similar structure, whether in a digital watch or in a 40-inch television. The liquid crystals are held between two layers of glass. A layer of transparent conductors on the liquid crystal side of the glass allows the liquid crystal layer to be manipulated. Polarized film layers are placed on the outside ends of the glass, one of which will face the viewer and the other will remain at the back of the display.

Polarizers alter the course of light. Typically, light travels outward in random directions. Polarizers present a barrier, blocking light from traveling in certain directions and preventing glare. The polarizers in an LCD are oriented at 90-degree angles from each other. With the polarizers in place alone, all light would be blocked from traveling through an LCD, but the workings of liquid crystals allow that light to come through.

The electrical current running through the liquid crystals controls their orientation. The rodlike crystals, without voltage, are oriented perpendicular to the glass of the screens. In this state, the crystals do not alter the direction of the light passing through. As voltage is applied, the crystals turn parallel to the direction of the screen. Like the polarized films, the conductors are oriented at 90-degree angles to each other, as are the crystals next to each screen (that is, crystals on one end are at a 90-degree angle from those on the other end). Between, however, the crystals orient in a twisting pattern, so light polarized in one direction will be redirected and turned 90 degrees when it emerges at the other end of the display. This is known as the twisted-nematic effect.

Thus, the voltage applied to the crystals controls the light coming through the LCD screen. At lower voltage levels, some, but not all, light is allowed through the display. By manipulating the intensity of the incoming light, the LCD can display in a gray scale.

Because liquid crystals are a state of matter, they exist only at a certain temperature: between the melting and freezing points of the material. Thus, LCD displays may have trouble working in extreme heat or extreme cold. One of the primary challenges of LCD display is finding materials that remain in liquid crystal forms at the temperatures in which the devices are likely to be used. Some of the challenge also lies in finding materials that may display better color or allow for lower energy consumption. However, materials that do one of these things better may make other features of a display worse. In some cases, a mixture of compounds for the liquid crystals in a device may be used.

Simple LCDs. The simplest LCD displays, such as calculators and watches, typically do not have their own light sources. Instead, they have what is known as passive display. In back of the LCD display is a reflective surface. Light enters the display and then bounces off the reflective surface to allow for the screen display. Simple LCDs are monochromatic and have specific areas (typically bars or dots) that become light or dark. While these devices are lower-powered, some do still use a light source of their own. Alarm clocks, for example, have light-emitting diodes (LEDs) as part of their display so that they can be seen in the dark.

Personal Computers and Televisions. For larger monitors that display complex images in color, the setup for an LCD becomes more complicated. Multicolor LCDs need a significant light source at the back of the display.

The glass used for more sophisticated LCD displays will have microscopic etchings on the glass plates at the front and back of the display. As with the polarizing filters and the conductors, the etchings are at 90-degree angles from each other, vertical on one plate and horizontal on the other. This alignment forms a matrix of points in each location where the horizontal and vertical etchings cross, resulting in what are known as pixels. Each pixel has a unique “address” for the electronic workings of the display. Many television sets are marketed as having 1080p, referring to 1,080 horizontal lines of pixels.

An active matrix (AM) display will have individual thin film transistors (TFTs) added at each pixel to allow for control of those sites. Three transistors are actually present at each pixel, each accompanied by an additional filter of red, green, or blue. Each of those transistors has 256 power levels. The blending of the different levels of those three colors (256³, or 16,777,216 possible combinations) and the number of pixels allows for the full-color LCD displays.

While light is displayed as a combination of red, green, and blue on screens, printing is typically done on a scale that uses cyan, magenta, yellow, and black as base colors. This accounts for some discrepancy between colors that appear on screen and those that show up on paper.

Applications and Products

Liquid crystals are used in displays for a number of products. Early uses included digital thermometers, digital wristwatches, electronic games, and calculators. As the power needed for an LCD display and resolution improved, LCDs came to be used in computer monitors, television sets, car dashboards, and cellphones.

Calculators and Digital Watches. Watches and calculators use what is known as a seven-segment display, wherein each of the seven segments that make up a number are “lit” or “unlit” to represent the ten digits. Looking closely at an LCD will reveal that most numbers come from seven segments, which can be lit to display the ten different digits. Without the polarizing layer, the display would not work. Placing the polarized layers in parallel on the surface would, for example, cause the outlines of all the numbers and other areas on the display to illuminate (appearing as 8s) and leave as blanks the rest of the display.

Early electronic games also used a segment display. Fixed places on the display would be either lit or unlit, allowing game characters to appear to move across the screen.

Temperature Monitors. Because of their sensitivity to heat, liquid crystals have been studied for their use as temperature monitors. Molecules in the smectic liquid crystal state rotate around their axes, and the angle at which they rotate (the pitch) can be temperature sensitive. At different temperatures, the wavelength of light given off will change. Some liquid crystal mixtures are fairly temperature sensitive, and so the mixture of the colors will change with relatively small changes in the temperature. Because of this, they can be used for displays such as infrared or surface temperatures.

Computer Monitors. LCDs have been, and will likely remain, the standard for laptop computers. They have been used for monitors since the notebook computer was introduced. Because of the low power consumption and thinness of the monitor, their use is likely to continue.

Television Sets. The workings of LCDs in televisions have been outlined in the foregoing section. As will be discussed further in the next section, LCDs have had a marked impact on television displays, both in the quality of displays in the home and in industry overall.

There are several developments that could affect LCD technology in the near future. One example is the development of LEDs for use as backlighting for LCDs. By using LEDs rather than a fluorescent bulb, as LCD technology now uses, LCDs can manifest greater contrast in different areas of the screen. Other areas of LCD development include photoalignment and supertwisted nematic (STN) LCDs.

Grooves are made in glass used for LCDs, but this has raised some concern about possible electric charges, reducing the picture quality. Additionally, photoalignment—a focus on the materials used to align the liquid crystals in the display—should ultimately allow for liquid display screens that are flexible or curved, rather than rigid (as are glass panels).

STN LCDs are modified versions of TN LCDs. Rather than twisting the crystals between the layers a total of 90 degrees, STN LCDs rotate the crystals by 270 degrees within the display. This greater level of twisting allows for a much greater degree of change in the levels of brightness in a display. At the same time, it presents a challenge because the response time for the screen is significantly slower.

Social Context and Future Prospects

Some of the concerns and problems with LCDs are being confronted by society as a whole. One concern is the high energy consumption of fluorescent lamps used by LCDs. In contrast, LED lights, which use less energy, are being used more and more in LCDs. There is concern, however, about the environmental hazards LEDs may create when they are disposed of in landfills. Another possibility is the use of carbon nanotubes, which would provide LCD backlighting but would use even less energy than LEDs.

Durability concerns may also come to play a role. The grooves in the glass necessary for high definition LCDs also lead to physical wear and tear on the product. Refining the technology further may produce more durable sets while also alleviating some of the concerns about electronics disposal. Future work on LCDs also will involve altering components to overcome picture quality and durability concerns. Given the prominence of the products that utilize liquid crystals, the technology is likely to be important for development for the foreseeable future. Experts predict the market will grow about 5 percent from 2022 to 2026.

Further Reading

Chandrasekar, Sivaramakrishna. Liquid Crystals. 2nd ed. New York: Cambridge UP, 1992. Print.

Chigrinov, Vladimir G., Vladimir M. Kozenkov, and Hoi-Sing Kwok. Photoalignment of Liquid Crystalline Materials. Chichester: Wiley , 2008. Print.

Collings, Peter J., and Michael Hird. Introduction to Liquid Crystals. New York: Taylor, 1997. Print.

Delepierre, Gabriel, et al. “Green Backlighting for TV Liquid Crystal Display Using Carbon Nanotubes.” Journal of Applied Physics 108.4 (2010). Print.

"Outlook on the Terminal LCD Displays Global Market (2022-2027)--Featuruing Bosch, Rexroth, Casio America, Control4 and Creston Electronics Among Others." ResearchandMarkets.com, 7 June 2022, www.businesswire.com/news/home/20220607005851/en/Outlook-on-the-Terminal-LCD-Displays-Global-Market-2022-to-2027---Featuring-Bosch-Rexroth-Casio-America-Control4-and-Crestron-Electronics-Among-Others---ResearchAndMarkets.com. Accessed 9 June 2022.